1. Field of the Invention
The invention is related to an electroluminescent display, and more particularly, to an electroluminescent display with improved lifetime.
2. Description of the Prior Art
In recent years, due to the successful development of organic materials, the organic light-emitting display (OLED) has been noticed in the display market because of its advantages such as simple structure, low working temperature, high contrast, and wide visual angle.
Referring to FIG. 1, it is a schematic diagram of the method of fabricating organic light-emitting elements on a substrate according to the prior art. Generally, an OLED 14 comprises devices fabricated on a substrate 16, which has a plurality of pixel regions 30 defined thereon. Each pixel region 30 comprises at least three sub-pixel regions 24, 26, 28 to produce light with different colors, such as red light, green light, and blue light. Each of the sub-pixel regions 24, 26, 28 comprises at least one switch device, such as a thin-film transistor 18, to control the operation of the corresponding sub-pixel region 24, 26, or 28. In addition, an organic light-emitting element 20 is disposed in each of the sub-pixel regions 24, 26, 28 and electrically connected to the thin-film transistor 18, so that electrons and holes can be recombined in the light-emitting layer 22 of the organic light-emitting element 20 to produce light when the sub-pixel region 24, 26, or 28 is turned on. Furthermore, light sources with different colors need to be fabricated with different organic light-emitting materials. Here a traditional OLED composed of three kinds of sub-pixel regions, the red, green, and blue sub-pixel regions, is used as an example for explanation. When fabricating the organic light-emitting elements 20 for producing red light, green light, and blue light individually, the red, green, and blue light-emitting layers 22 need to be formed separately in the sub-pixel regions 24, 26, 28 respectively. For example, a first, a second, and a third shadow masks are used separately to fabricate the red, green, and blue light-emitting layers 22 in the corresponding sub-pixel regions 24, 26, 28, wherein each of the first, second, and third shadow masks has a plurality of openings for defining the positions of the red, green, or blue sub-pixel regions 24, 26, or 28. As indicated in FIG. 1, the green light-emitting layers 22 can be formed through the openings 12 of a first shadow mask 10 on the substrate 16, and then the light-emitting layers 22 with other colors can be formed through the openings of a second or a third shadow mask on the substrate 16 of the OLED 14.
Referring to FIG. 2, it is a schematic diagram of the shadow mask 10 for fabricating the organic light-emitting element 20 shown in FIG. 1 according to the prior art. The traditional shadow mask 10 comprises a plurality of openings 12. While the organic light-emitting elements 20 are being fabricated, the light-emitting layers 22 are formed on the substrate 16 through the opening 12 of the shadow mask 10. Therefore, the size and distance of the openings 12 determine the distance of any two adjacent organic light-emitting elements 20, and moreover control the size and density of the sub-pixel regions 24, 26, 28. Currently, the shadow mask 10 is mostly fabricated by a photolithography-etching process or an electroforming process. Under the technology limit of these processes, a minimum distance B exists between two neighboring openings 12 in order to make the shadow mask 10 have enough strength, which becomes the bottleneck of the progress in increasing the light-emitting areas of the sub-pixel regions 24, 26, 28.
Moreover, since the lifetime of a general organic light-emitting element will be shorten following the increase of the operating current, and the blue light organic light-emitting element has the shortest lifetime among red, green and blue light organic light-emitting elements, how to adjust the location allocation of these sub-pixel regions to improve the density of the pixel regions and the lifetime of the display is still an important topic for the industry in need of researching.